The first organic light-emitting diode (OLED) was disclosed by Tang et. al. (U.S. Pat. No. 4,356,429) and Tang et. al. (Appl. Phys. Lett. 1987, 51, 12, 913). Subsequently, device architecture and emissive materials for OLED applications have been intensively studied. OLEDs allow devices that are (1) ultra-thin; (2) self-emissive; (3) use low operating voltage with high efficiency; and (4) display high contrast and resolution.
Phosphorescent materials are the primary direction of emissive material development, because these OLED devices generate 25% singlet and 75% triplet excitons. Devices fabricated from phosphorescent materials generally display efficiencies that are higher than devices fabricated from fluorescent materials. Platinum complexes are a class of emissive materials that offer high emission quantum efficiency and good thermal stability. High performance OLEDs have been formed using platinum(II) complexes. (Yan et al., Appl. Phys. Lett. 2007, 91(6) 063508; Che et al., Chemistry—A European Journal 2010, 16(1), 233)
Producing blue-emitting phosphorescent materials has proven to be challenging. However, high performance and long lifetime blue-emitting OLEDs have not been fabricated from platinum(II) complexes. To tune the emission color of platinum(II) complexes, the appropriate conjugation length of the ligand is essential. Neutral blue-emitting Pt(II) complex were prepared by coordinating two bidentate ligands to a Pt(II) center, resulting in the first blue-emitting Pt(II) complexes. (Brooks et al., Inorg. Chem. 2002, 41, 3055; and Unger et al., Angew. Chem. Int. Ed. 2010, 49, 10214) However, binding forces between these bidentate ligands and the platinum(II) center in these complexes are weaker than the binding forces of complexes containing one tetradentate ligand bound to a Pt(II). The devices that form these bis-bidentate ligand Pt(II) complexes display lifetimes and stabilities that are inferior to device tetradentate Pt(II) complexes. Ligand systems having more than two aryl groups, coupled by a non-conjugated unit, do not provide emission maxima less than 480 nm. (U.S. Pat. No. 7,026,480, U.S. Pat. No. 6,653,654). Blue-emitting materials have not been made from this type of tridentate or tetradentate ligands. Efforts directed to tetradentate blue-emitting Pt(II) materials where conjugation groups connect aryl groups of the ligands, have failed to generate complexes with emission maxima less than 480 nm. (U.S. Pat. No. 7,361,415, U.S. Pat. No. 7,691,495 and U.S. Published Patent Application 2007/0103060 A1)
N-heterocyclic carbenes (NHC) are strong σ-donors but poor π-acceptors. Using cyclometalated Pt(II) complexes, Meyer et al., Organometallics, 2011, 30 (11), 2980 discloses Pt(II) complexes with shortened ligand π-conjugations, but does not report of the emission spectra.